This invention relates to a deflection device used in a cathode ray tube, and more particularly, to a deflection device suitable for high frequency deflection.
The horizontal deflection frequency usually employed in cathode ray tubes, for example, color cathode ray tubes, is 15.75 KHz. However, in tubes such as display tubes, where high resolution and improved visual recognition characteristics are required, conditions are now more commonly met that require the use of higher horizontal deflection frequencies, for example, 25 KHz and 31 KHz. In particular, some display tubes used in computer aided design and computer aided manufacturing applications, in which computers are used for technical design or for production control, operate with a horizontal deflection frequency of 64 KHz.
When deflection devices are operated with the high horizontal deflection frequencies referred to above, the following problem arises.
Specifically, the horizontal magnetic field produces eddy currents in the core and horizontal deflection coil constituting the deflection device, and these currents generate heat.
For example, the curve A shown in FIG. 7 plots horizontal deflection frequency vs. temperature rise. According to this curve, when the above display device for a 14-inch 90.degree. deflection display tube is operated at the conventional horizontal deflection frequency, the temperature rise .DELTA.T of the core is about 20.degree. C. However, if it is operated at 64 KHz, this .DELTA.T is about 70.degree. C. The heat proof temperature of a polypropylene molt at which mold deformation may occur is about 105.degree. C. When a deflection device for a 14-inch 90.degree. deflection display tube is operated at 64 KHz in circumferential air at a temperature of 50.degree. C., the core rises in temperature by about 70.degree. C. as shown in FIG. 7. Thus the temperature in its vicinity, i.e. the temperature of the mold approaches 120.degree. C. This causes deformation of the polypropylene mold. This is a serious problem from the point of view of performance and reliability. Of course, this can be overcome if a material of even higher thermal resistance is used, but this results in a large increase in material cost and/or required machining precision. This is very disadvantageous from the point of view of mass production. Japanese Patent Application Laid-open No. 59-186239 discloses a technique in which Litz wire is used to reduce the stray capacitance of the deflection coil to reduce the temperature rise due to eddy current losses. However, it has not proved possible to reduce heat generation sufficiently by this means alone.
In this case, the heat generated in the coil may be reduced. However, the heat generated in the core, and in the magnetic member attached between the core and the mold for adjusting the deflection field, cannot be limited. This magnetic member has a relatively low electric resistance due to the use of a silicon steel plate therein. This low resistnce causes an increase in the eddy current in the magnetic member according to a higher deflection frequency.